Debian

Debian is a widespread Linux distribution, that's used mainly on desktop machines, but many people have started using it lately as a web server Operating System as well, due to the stability and security it offers. The latter is a direct result the fact that Debian is developed and supported by countless qualified volunteers worldwide, therefore any patch that's necessary to fix a particular issue is released within a couple of days. The same community will also help you with any question or problem that a user may ever experience, because they can be contacted via e-mail using mailing lists and a qualified response will be sent in no more than 15-20 min. The advantage here is that all your queries will be addressed by the same people who have developed the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you won't have to pay any license fees to use it and you can personalize it as much as you want or need. This is valid for the Operating System as well as the 30.000+ software packages which are readily available for it.